(1) Field of the Invention
This invention relates to a pressure fixing device for effecting fixing of a toner image on a supporter only by application of pressure at the supporter as the supporter moves between a pair of fixing rolls.
(2) Description of the Prior Art
In electrophotographic copying apparatus of the dry type, printers, facsimile system, etc., a pressure fixing device is used, for example, for fixing a toner image formed on a supporter, such as paper, following developing or transfer-printing. Various types of pressure fixing device are available, but a device having a pair of rollers, such as the one shown in FIG. 4 of Japanese Utility Model Laid-Open No. 164261/81, or a device having three rolls, such as the one shown in Japanese Patent Publication No. 48315/80, is usually used.
In this type of pressure fixing device, means is usually provided for causing the axes of the pair of rolls to cross each other to compensate for a deflection that might occur between the rolls maintained in pressing engagement with each other and obtain an axially uniform distribution of pressure, as described in Japanese Patent Publication No. 6911/80 and Japanese Patent Laid-Open No. 4075/80.
In this type of pressure fixing device, it is essentially impossible to render totally uniform in an axial direction the pressure distribution between the fixing rolls and the surface coarseness of the rolls, and the line pressure between the fixing rolls is high (10-25 kg/cm.sup.2), so that the sheets handled tend to develop wrinkles at a higher rate than in fixing devices of the hot roll type. Particularly when there is a high degree of variation in the amount of toner adhering to the supporter axially thereof, the sheet tends to develop wrinkles. In the event that the toner supporter is transfer-printing sheet, such as ordinary paper, the paper would be fed between the rolls in waved condition due to the moisture contained therein when the humidity is high, thereby causing the paper to wrinkle.
To avoid wrinkle formation in the paper, a proposal has been made, as disclosed in Japanese Patent Publication No. 6911/80 and Japanese Patent Publication No. 3943/82, for example, to set the peripheral velocity of the fixing rolls at a level lower than the velocity at which the paper is moved, to loosen the paper from its carrier before the paper is fed into the nip between the fixing rolls. However, pressure fixing devices of the prior art have the trouble that, because the paper is loosened greatly, an impact of high order is produced when the paper is bitten by the fixing rolls and the toner image tends to be disturbed by the force of impact. Moreover, when the leading end portion of a copy sheet is caused to be uniformly bitten by the pair of pressure applying rollers by keeping the paper guide in one plane and parallel to a line of contact between the pair of pressure applying rollers, as described in Japanese Patent Publication No. 3943/82, an impact of high magnitude is produced when the sheet is bitten by the rollers and the force of impact is particularly high if there is a variation in the amount of the toner adhering to the sheet widthwise thereof.
Other proposals have also been made to avoid wrinkle formation. For example, Japanese Utility Model Laid-Open No. 39063/82 discloses the use of a guide member of a shape in which the forward end edge projects nearer to the rolls in its central portion than in opposite sides. In Japanese Utility Model Laid-Open No. 60159/82, a paper guide is disclosed which is curved at its forward end to provide a downwardly inclined surface substantially corresponding in configuration to the peripheral surface of the upper roller. The proposals would suffer the disadvantage that the shape of the paper guide is complex and might not suit practical use.